Enable full-text search using default Lunr search functionality

[aschrijver]

Lunr is the default search engine that is integrated with the minimal-mistakes theme, and we should enable that first.

Other options are Algolia and Google, but Algolia requires a paid account ($29/month) to have more than one team member that is able to manage the index. Google integration comes with google tracking, and should be avoided.

Lunr is probably rather limited, but offers full-text indexing of the website, though this can come at performance penalties. Information on the integration is here:

• https://mmistakes.github.io/minimal-mistakes/docs/configuration/#lunr-default

Note: Need to check privacy implications and update Privacy Policy accordingly (#31)